The energy density of
coal, i.e. its heating value, is roughly 24megajoules per
kilogram.
The energy density of coal can also be expressed in
kilowatt-hours, the units that electricity is most commonly sold in, per units
of mass to estimate how much coal is required to power electrical appliances.
One kilowatt-hour is 3.6 MJ, so the energy density of coal is 6.67 kW·h/kg. The typical thermodynamic efficiency of coal power
plants is about 30%, so of the 6.67 kW·h of
energy per kilogram of coal, 30% of that—2.0 kW·h/kg—can
successfully be turned into electricity; the rest is waste heat. So coal power
plants obtain approximately 2.0 kW·h per
kilogram of burned coal.
As an example, running one 100-watt lightbulb for one year
requires 876 kW·h (100 W × 24 h/day × 365
day/year = 876000W·h = 876 kW·h). Converting this power usage into physical coal consumption:
It takes 325 kg (714 lb) of coal to power a 100 W lightbulb
for one year. One should also take into account transmission and distribution
losses caused by resistance and heating in the power lines, which is in the
order of 5–10%, depending on distance from the power station and other factors.